1. Field of the Invention
This invention relates to photopolymerizable or radiation-polymerizable resin compositions which can be developed with an aqueous alkaline solution.
2. Description of the Prior Art
In recent years, so-called dry film resists comprising a layer of photopolymerizable resin sandwiched between a supporting film and a protective film are being widely used as photoresists for the fabrication of printed wiring boards. According to the nature of the photopolymerizable resin, there are two types of dry film resists. One of them is the solvent-developing type in which unexposed areas are removed by means of a chlorine-containing organic solvent, and the other is the alkali-developing type in which unexposed areas are removed by means of an aqueous alkaline solution. However, the recent trend is towards the use of alkali-developing type dry film resists because of their little influence on the environment and their low manufacturing cost.
Typically, an alkali-developing type dry film resist having a supporting film and a protective film is used in the following manner: While the protective film is being separated from the dry film resist, its photopolymerizable resin surface is heat-laminated to a copper-clad laminate. Then, a photo tool is brought into contact with the supporting film surface and the dry film resist is exposed to ultraviolet radiation or the like so that desired areas thereof may be cured. After the supporting film is separated, the resist is developed with an aqueous solution of a weak alkali such as sodium carbonate to remove the unexposed areas and thereby form a circuit pattern of resist. In the copper through-hole process, the copper surface is then subjected to etching. Thereafter, the cured resist is stripped with an aqueous solution of a strong alkali such as sodium hydroxide to obtain a printed wiring board. In the solder through-hole process, the resist-coated laminate is subjected to copper plating and solder plating. Thereafter, the cured resist is stripped with an aqueous solution of a strong alkali and the exposed copper is etched away to obtain a printed wiring board.
It is a matter of course that, when used in the above-described processes, dry film resists have a sufficient degree of resistance for use as etching resists or plating resists. Up to this time, various investigations have been made for the main purpose of improving such resistance.
However, many of the conventional alkali-developing type dry film resists have had the disadvantage that the time required to strip the cured resist with an aqueous solution of a strong alkali (i.e., stripping time) is long. Such a long stripping time causes the copper surface of the substrate to be oxidized and discolored by the action of the alkali, thus exerting an adverse influence on subsequent steps such as etching. Moreover, in the solder through-holes process, solder is dissolved in the aqueous solution of strong alkali, so that the thinning or breaking of patterns may occur and/or fusing may become impossible.
On the other hand, most of the conventional alkali-developing type dry film resist having a short stripping time have had the disadvantage that the stripped pieces obtained by treating the cured resist with a strongly alkaline stripping solution are easily dissolvable in the stripping solution. Where the stripped pieces of the cured resist are easily dissolvable in the stripping solution, they easily swell to form a gel while staying in the stripping tank, and the resulting gel frequently clog the meshes of the filter provided in the stripping machine for the purpose of capturing stripped pieces. As a result, the flow rate of the circulating stripping solution is reduced, so that it may become impossible to strip the cured resist and/or the stripping solution may flow over the edge of the stripping tank to contaminate the surroundings thereof.
Moreover, most of the conventional alkali-developing type dry film resists having a short stripping time have had the disadvantage that the size of the stripped pieces obtained by treating the cured resist with a strongly alkaline stripping solution is too large or too small. If the stripped pieces are too large (i.e., larger than 45 mm.times.45 mm), they tend to cling to the carrying rollers of the automatic stripping machine and may develop machine trouble. In Japanese Patent Laid-oipen No.10235/'89, therefore, an attempt has been made to reduce the size of stripped pieces by using a specific corsslinkable monomer. However, if stripped pieces are fine (i.e., smaller than 2 mm.times.2 mm), they tend to pass through the meshes of the filter provided in the stripping machine for the purpose of capturing stripped pieces. As a result, such stripped pieces enter the circulating pump and the spray nozzle, thus interfering with the circulation of the stripping solution. Moreover, since such stripped pieces are not removed but stay in the stripping solution for a long period of time, the stripping solution deteriorates rapidly and/or the stripped pieces are dissolved therein, thus necessitating frequent replacement of the stripping solution.
Thus, conventional dry film resists have been unable to achieve a desirable combination of properties including a short stripping time, a proper stripped piece size, and the insolubility of stripped pieces in the stripping solution.